In some cases, the effect of less-abundant isotopes might cause you to use an average molecular weight rather than one based on the most abundant isotopes. When the molecular structure of the target compound contains large numbers of certain elements, the less abundant isotopes become significant. This situation might result in a shift in the mass peaks from their expected m/z values.
For example, the most abundant isotope of chlorine is 35Cl. However, 37Cl occurs with a natural abundance of 24.47 percent. When a compound contains four chlorine atoms, its molecular ion is two mass units greater than that expected from a calculation based solely on 35Cl. Using chlorine’s average atomic weight (35.453 Da), you can correctly identify the molecular ion. Also, you observe a distribution of molecular ions across eight mass units from molecules containing between zero and four 37Cl atoms.